Integrated circuit devices having high levels of device integration frequently utilize signal lines and electrically conductive device structures that are spaced closely to each other. In order to reduce parasitic coupling capacitances between the closely spaced signal lines and device structures, electrically insulating materials having relatively low dielectric constants have been considered as materials for electrically isolating signals lines and device structures from each other. The use of insulating materials with low dielectric constants improves device performance by reducing the RC time constants of signal lines and conductive device structures and thereby supporting higher speed switching.
One commonly used electrically insulating material having a relatively low dielectric constant is SiCOH. Unfortunately, SiCOH may be etched at a relatively high rate because it is relatively porous compared to other insulating materials such as SiO2. Hard mask layers have frequently been utilized as etch masks in order to limit over-etch of SiCOH materials. Nonetheless, lateral over-etching and undercutting of SiCOH may occur even when hard mask layers are used. This lateral over-etching is illustrated by FIG. 1. In particular, FIG. 1 illustrates an electrically insulating layer 20b formed of a material having a relatively low dielectric constant (e.g., SiCOH) and a hard mask layer 20a formed on the electrically insulating layer 20b. A photolithographically defined etching step is performed to define an opening that extends through the hard mask layer 20a and the underlying electrically insulating layer 20b. As illustrated, the etching step may result in the formation of an undercut region U having a sidewall that is laterally recessed relative to the opening in the hard mask layer 20a. The opening is then filled with an electrically conductive region 10, which may have reduced reliability as a result of the undercut region U. In particular, the presence of the undercut region U may result in the formation of a void (not shown) in the electrically conductive region 10.